The present invention relates generally to stents which are implantable or deployable in a vascular or endoluminal location within the body of a patient to maintain the lumen open at that location, and more particularly to improvements in stent coatings for biocompatibility and in methods for applying such coatings.
Stents are expandable prostheses employed to maintain narrow vascular and endoluminal ducts or tracts of the human body open and unoccluded, such as a portion of the lumen of a coronary artery after dilatation of the artery by balloon angioplasty. While vascular usage is frequently discussed in this application, it will be understood by those skilled in the art that stents having the characteristics and features of the present invention may be implanted in other ducts or tracts of the human body to keep the lumen open, such as in the tracheo-bronchial system, the binary hepatic system, the esophageal bowel system, and the urinary tract system.
In the case of an occluded coronary artery, for example, the original blockage is typically attributable to fatty deposits or plaque on the inner lining of the vessel. A different mechanism, however, produces a new blockage after an angioplasty procedure is performed to compress the deposits against the inner lining of the vessel, as by use of balloon angioplasty, or to virtual entirely remove the deposits, as by use of laser angioplasty or rotational cutting. The blood vessel wall is subjected to trauma by any of these procedures, which results in hyperplasia of the neointima, i.e., a rapid proliferation of muscle cells in the affected region of the wall, to cause restenosis and re-occlusion of the vessel lumen in a significant percentage of angioplasty patients within a period of from three to six months following the initial procedure.
To avoid this re-occlusion and to maintain the lumen of the vessel open, it is customary procedure to install a stent at the site in the vessel where the angioplasty was performed. The stent is deployed by radial expansion under pressure exerted by the inflating balloon of a balloon catheter on which the stent is mounted, to engage the inner lining or surface of the vessel wall with sufficient resilience to allow some contraction but also to provide a degree of stiffness to resist the natural recoil of the vessel wall following expansion.
The presence of the stent itself in the bloodstream, however, promotes thrombus formation and clotting as blood flows through the vessel. This, too, can result in sufficient blockage of the coronary artery to produce an infarction. Thrombus formation and clotting at the inner lumen of the stent, and fibrosis and restenosis at the site of the vessel wall where the angioplasty was performed and the outer surface of the stent is now engaged, can be significantly reduced by application of appropriate acutely acting drugs in the locality of the stent. In the past, some difficulty has been encountered in providing a stent surface which is suitable for retention of the necessary drug(s) to achieve those purposes.
Additionally, the composition of the stent, or at least its surface material which is exposed to blood, other body fluid, or tissue, is a factor in the patient's tolerance of the stent in the vascular or endoluminal duct. Of course, the stent must have a composition which is biocompatible with the blood, fluids and tissue of the patient's body. But fully five percent of the human population exhibit allergies to chrome, nickel, and even medical grade 316L stainless steel (about 20% nickel)--materials of which stents are commonly composed. Special biomaterial coatings can provide surfaces which are non-allergenic.
Another important consideration in stent selection and usage is its mechanical strength, particularly in applications where the small size of the duct severely limits the physical dimensions of the stent, such as in a coronary artery. The diameter of the stent and the thickness of its wall or wire must be maintained at a minimum, and yet still offer sufficient mechanical strength to resist the natural recoil of the vessel wall following implantation and to keep the lumen of the vessel open. A steel stent having a wall thickness of from 40 to 50 microns (micrometers, .mu.m) is feasible for use in the coronary artery and with proper design can be highly flexible when crimped on a balloon while providing significant mechanical strength when deployed. But the small diameter and thin wall of such a stent may not provide enough retention force for film attachment when the stent is crimped onto the balloon, particularly if the stent surface is polished as is frequently the case.
Also, a stent of such small dimensions is virtually not visible on X-ray fluoroscopy as it is being implanted in the patient's body, or afterward when the implant site is examined during patient follow-up, because of its low X-ray absorption.
Therefore, the principal aim of the present invention is to provide a method for manufacturing a small diameter stent which has excellent visibility on X-ray fluoroscopy, as well as high retention force when mounted onto a balloon, and excellent biocompatibility with low thrombogenicity when implanted in a vessel.
A related aim is to provide a method of fabricating a stent in which a ceramic-like structure is applied as an outer layer to a base metal, where the two differ in tensile strength and physical characteristics.
Another important aim of the invention is to provide a stent and method of manufacture thereof in which a suitable coating is provided on the exposed surfaces of the stent to measurably reduce tissue irritation. The reduction in tissue irritation also reduces the traumatic response that produces rapid proliferation of the tissue, and hence, impedes the restenosis attributable to that mechanism. The coating improves thrombogenicity and provides a surface region which may be used to carry an additional biodegradable layer impregnated with anti-fibrotic and anti-thrombotic drugs, for example, which are released to avoid responses tending to initiate a reblockage of the vessel in which the stent is implanted, especially the coronary artery.
Still another objective of the present invention is to provide a stent with a special coating that resists occlusion of a blood vessel at the implant site attributable to mechanical stress-induced hyperplasia of the intimal and neointimal region of the vessel wall, and stent-induced clotting and thrombus formation.
Yet another aim of the invention is to provide a stent which has improved radiopacity for X-ray fluoroscopy viewing without increasing the physical dimensions of the stent.